1. Field of the Invention
The present invention relates to photonic pacemakers designed for compatibility with MRI diagnostic equipment, and to other opto-electric medical stimulation and sensing equipment. More particularly, the invention concerns an MRI-compatible medical device with passive generation of optical sensing signals.
2. Description of Prior Art
By way of background, MRI compatible pacemakers for both implantable and wearable use have been disclosed in copending application Ser. Nos. 09/864,944 and 09,865,049, both filed on May 24, 2001, and copending Ser. Nos. 09/885,867 and 09/885,868, both filed on Jun. 20,2001. In the aforementioned copending patent applications, whose contents are fully incorporated herein by this reference, the disclosed pacemakers feature photonic catheters carrying optical signals in lieu of metallic leads carrying electrical signals in order to avoid the dangers associated with MRI-generated electromagnetic fields. Electro-optical and opto-electrical transducers are used to convert between electrical and optical signals. In particular, a laser diode located in a main pacemaker enclosure at a proximal end of the photonic catheter is used to convert electrical pulse signals generated by a pulse generator into optical pulses. The optical pulses are carried over an optical conductor situated in the photonic catheter to a secondary housing at the distal end of the photonic catheter, where they are converted by a photo diode array into electrical pulses for cardiac stimulation.
Despite the advances in pacemaker MRI compatibility offered by the devices of the above-referenced copending applications, there remains a problem of how to provide physiological sensing capability in such devices. In a conventional pacemaker, there are direct electrical pathways between the power source (typically a lithium battery) and the circuitry responsible for generating stimulation and sensing signals. Because the circuitry has low power requirements, battery life can be prolonged for relatively long time periods. With a photonic pacemaker as contemplated by the above-referenced copending applications, the power requirements are greater due to the high current demands of the laser diodes used for optical signal generation, and the inefficiencies associated with opto-electrical and electro-optical conversion. Sensing signals are especially problematic. Most devices used to sense physiological conditions rely on electrical signals. These include xe2x80x9cRxe2x80x9d wave sensors for sensing electrical activity in the heart, partial oxygen (pO2) sensors, temperature sensors, etc. Such electrical signals are very low in power (e.g., less than one milliwatt) and cannot be directly used to drive conventional electro-optical transducers, such as laser diodes or light emitting diodes. Instead, they need to be amplified or otherwise conditioned in order to boost their strength to a point where they can be converted into optical form. Only then can the sensing signals be transported over the photonic catheter to upstream sensing circuitry. It will thus be appreciated that a source of electrical power is required at the distal end of the photonic catheter. This poses a non-trivial design problem in the photonic medical equipment art.
The foregoing problem is solved and an advance in the art is provided by a novel system and method for passive optical sense signal generation in a photonic pacemaker or other opto-electric medical device. The system is adapted to operate at the distal end of an implantable photonic catheter having one or more optical conductors for conducting light energy in two directions between electronics at a proximal end of the catheter and electrical stimulation and sensing components at the catheter distal end. The latter may include pacing electrodes, sensing electrodes, partial oxygen sensors, temperature sensors, etc.
An optical unit receives a light input delivered from the catheter proximal end by one (or more) of the optical conductors. The optical unit directs a first portion of the light input as a first light output to an opto-electrical converter for conversion into electrical stimulation signals and directs a second portion of light input as a second light output to one or more optical modulators that modulate the second light output under applied electrical signals. An electrical circuit is connected to the device""s sensing component(s). The electrical circuit generates electrical sense signals corresponding to one or more sensed physiological conditions and provides the signals to the optical modulator(s). This results in modulation of the second light output into optical sense signals that are transmitted over one or more of the catheter""s optical pathways to the catheter proximal end.
In preferred implementations of the invention, the optical unit comprises a prismatic optical homogenizer having a first end adapted to receive the light input and a second end having a partially reflective coating thereon. The partially reflective coating allows the portion of the light input to pass as the first light output and reflects the second portion of the light input back to the first end as the second light output. The optical modulators can then be mounted on the first end of the optical homogenizer. The partially reflective coating can be formed as a partially reflective material covering all of the second end. Alternatively, it can be formed as a fully reflective material covering a peripheral portion of the second end while leaving uncovered a central portion of the second end for transmitting the second portion of the light input. The optical modulator can be formed from a crystalline material having electrically controllable optical properties. Exemplary materials include crystal structures selected from the group consisting of lithium niobate, indium phosphide and gallium arsenide. The electrical circuit is preferably adapted to condition a physiological sense signal generated via interaction with body tissue into the electrical sense signal. The electrical circuit is preferably also adapted to obtain electrical power from the transmitted light output. Additionally, the electrical circuit may be adapted to convert the electrical sense signal form analog to digital form.
The invention further contemplates, respectively, a photonic pacemaker and an opto-electric medical stimulation system having the above-summarized optical sense signal generating functionality therein.